The present subject matter generally relates to electrical lead assemblies in devices such as electric lamps for providing an electrical path through a hermetic press, pinch, or shrink seal formed in a vitreous material such as fused silica or hard glass.
In certain devices, it is often necessary to provide an electrically-conducting path through a pinch or shrink seal formed in a vitreous material. For example, in devices such as electric lamps, e.g., halogen incandescent filament bulbs and high intensity discharge (“HID”) arc tubes, a light emitting chamber is formed from a vitreous material having one or more pinch seals that hermetically seal the chamber. In such lamps, one or more electrically-conducting paths from the interior of the chamber to the exterior of the chamber are typically formed by positioning an electrical assembly in one or more of the portions of the tube, and “pinching” the tube to form a hermetic seal around a portion of the assembly. The electrical lead assembly typically includes a metallic foil having electrically conducting leads mechanically secured to the foil and extending from each end thereof. The assembly is positioned so that the foil forms the electrically conducting path through a portion of the vitreous material that has been pinched or shrunk together to form a hermetic seal.
Although any suitable material may be used, typically, the foil in such electrical lead assemblies is formed from molybdenum because of its stability at high temperatures, relatively low thermal expansion coefficient, good ductility, and sufficient electrical conductivity. However, molybdenum oxidizes rapidly when exposed to oxygen at temperatures greater than about 350° C. Since the foils in electrical lead assemblies in electric lamps are often exposed to temperatures greater than about 350° C., the metallic foil may be highly susceptible to oxidation resulting in a breach of the electrical path or the gas-tight integrity of the hermetic seal resulting in lamp failure. Typically, a molybdenum foil exposed to a reactive atmosphere will not oxidize appreciably below about 350° C. At temperatures greater than about 350° C., the rate of the reaction between the oxygen in the surrounding atmosphere and the molybdenum foil greatly increases resulting in corrosion of the foil and a substantial reduction in the useful life of the lamp. Areas particularly susceptible to such oxidation include the spot weld connecting the outer lead to the foil and the area on the foil adjacent the outer lead.
FIG. 1a is a schematic representation of a conventional arc tube for a high intensity discharge lamp. Referring to FIG. 1a, the arc tube 100 is formed from light transmissive material such as quartz. The arc tube 100 defines a chamber 110 formed by pinch sealing the end portions 115, 120. An electrode assembly 122, 124 is sealed within each end portion 115, 120 to provide an electrically-conducting path from the interior of the chamber 110 to the exterior of the chamber through each end portion 115, 120. Each electrode assembly 122, 124 for a high intensity discharge arc tube 100 typically includes a discharge electrode 125, 130, electrode leads 140, 135, metallic foils 145, 150, and outer leads 155, 160. The electrode leads 135, 140 and the outer leads 155, 160 are typically connected to the metallic foils 145, 150 by spot welds.
FIG. 1b is an illustration of the cross-section of a typical metallic foil 145, 150 in an electrical lead assembly 122, 124. As shown in FIG. 1b, the typical foil 145, 150 is shaped in cross-section so that the thickness of the foil is greatest at the lateral center thereof, and reduces outwardly to each of the longitudinal edges. This shape has been found to reduce residual strain in the vitreous material that has been compressed around the foil during the high temperature pinching process and subsequently cooled. In a typical electrical lead assembly for an electric lamp, the foil may have a width of about 2 to 5.5 mm with a centerline thickness of about 20 to 50 μm and an edge thickness of about 3 to 7 μm. For example, a foil having a width of about 2.5 mm would typically have a centerline thickness of about 24-25 μm and an edge thickness of about 3 μm.
The assemblies 122, 124 are positioned in the end portions 115, 120 so that the foils 145, 150 are pinched between the compressed portions of the end portions 115, 120 forming the hermetic pinch seals. The assemblies 122, 124 provide the electrically conducting paths through the each end portion 145, 150 with the relatively thin foils 145, 150 providing a current path through the hermetically sealed pinch regions.
The electrode lead assemblies provide a point of failure in such lamps due to corrosion, e.g., oxidation, of the metallic foils when exposed to corrosive agents such as oxygen at high temperatures. This is primarily a problem for lamps that are operated in air, without an outer jacket, such as high wattage metal halide “sports” lamps, ultraviolet exposure lamps, HID projection light sources, and numerous incandescent tungsten halogen light sources. For example, the assemblies 122, 124 are particularly susceptible to oxidation at the outer portion of the foil 145, 150 adjacent the outer lead 155, 160 due to the exposure of this portion of the foil to oxygen or other corrosive agents during operation of the lamp. The oxidation may progress inward placing a significant amount of stress on the pinch seal. The stress may be evident from Newton rings or passageways which appear at the point at which the leads are welded to the molybdenum foil. Eventually, the electrical path may be breached or the pinch seal may crack causing the lamp to fail.
One reason for this failure is that during the formation of a pinch seal or vacuum seal with a vitreous material such as quartz, the quartz does not completely seal to the relatively thicker outer and inner lead wires, due at least in part to the relatively high viscosity of the quartz. Microscopic passageways may also be formed along the outer leads 155, 160 and also along the outer edge of the foliated portion perpendicular to the transverse axis of the lamp due to the substantial difference in the coefficient of thermal expansion of the quartz compared to that of the refractory metal outer lead wire, which is typically tungsten or molybdenum.
Another reason for this failure may also be the result of two mechanisms. First, as the molybdenum foil, wire or weld junction oxidizes, its resistance increases, leading to a further ohmic heating and higher temperatures and higher oxidation rates, eventually “burning” through the molybdenum material. Second, as the molybdenum foil, wire or weld junction oxidizes, molybdenum oxide products form. These oxides are generally less dense than the molybdenum metal materials, and the resulting expansion forces the quartz-to-metal or glass-to-metal seal apart, causing cracks and breaks. This second mechanism may also expose additional areas of molybdenum materials to air oxidation. Another common problem in pinch and shrink seals is the phenomenon referred to as “shaling.” In shaling, uneven stresses in the pinch or shrink area may be caused by the adherence of the quartz to the molybdenum metal surfaces thereby resulting in minute cracks. These cracks severely weaken the glass and may lead to failure of the respective lamp from very moderate strains.
Efforts have been made in the past to prevent the oxidation of molybdenum foils in electrical assemblies that may be exposed to oxygen at high temperatures. For example, it has been proposed to reduce oxidation by coating the molybdenum foil with oxidation-protective materials such as phosphides (U.S. Pat. No. 5,387,840), aluminides, lead oxide, silicon nitride, alkali metal silicate and chromium (U.S. Pat. No. 3,793,615). Another conventional practice for protecting the molybdenum foil involves filling the open end of the pinch or shrink area with a low-melting antimony borate glass. Yet another conventional practice includes protecting the outer lead with a platinum cladding. The utility of the aforementioned prior art approaches are marginally adequate and/or expensive; however, none of these prior art approaches includes the application of glassy films. A need, therefore, remains for oxidation-protected metallic foils for use in electrical lead assemblies for providing electrically-conducting paths through pinch seals in vitreous material and that can be exposed to high operating temperatures. It is therefore an object of the present subject matter to provide electrical lead assemblies that obviate the deficiencies of the prior art.
One embodiment of the present subject matter provides a means of protecting metallic foils and outer lead wires in electrical lead assemblies of electric lamps from oxidation through the application of a coating containing a refractory “abhesive” such as, but not limited to, boron nitride to the surface of the metallic foil or to the lead wire or to the foil-lead junction. An abhesive is generally a material having the capability of resisting adhesion.
Another embodiment of the present subject matter utilizes high temperature of the pinch process itself to fuse a “green” formulation of silica onto complete lead assemblies; thus protecting the foil, the lead wire and the critical weld junction with a continuous film of dense silica. An example of a green formulation is described in parent and co-pending U.S. patent application Ser. No. 11/545,469, filed Oct. 11, 2006 which is a divisional application of and claims priority to U.S. patent application Ser. No. 10/702,558, filed Nov. 7, 2003, now U.S. Pat. No. 7,153,179, each of which are incorporated herein in their entirety.
Yet another embodiment of the present subject matter prevents or eliminates “shaling” in which uneven stresses in the pinch area are caused by the sticking of the vitreous material or quartz to the molybdenum or other metal surfaces.
One embodiment of the present subject matter provides a method of protecting a portion of a metallic foil from corrosion comprising coating a portion of the foil with a film comprising silica and applying a refractory abhesive to a portion of the film, each step occurring prior to forming a pinch seal. Another embodiment of the present subject matter is a novel method of providing an electrical connection through a pinch or shrink seal formed in a quartz or glass body. This method may comprise providing a quartz or glass body having at least one open end and providing an electrical lead assembly comprising a metallic foil. The method may also include applying a coating comprising a refractive abhesive to at least a portion of the metallic foil, positioning the electrical lead assembly in an open end of the body, and pinch or shrink sealing the open end of the body so that the quartz or glass of the body forms a hermetic seal around the metallic foil of the electrical lead assembly.
A further embodiment of the present subject matter provides a method of preparing an electrode lead assembly. The method may comprise providing an electrode lead assembly comprising a metallic foil and immersing at least a portion of the electrode lead assembly in a silica colloidal mixture. The method may also include removing the assembly from the mixture and coating the dried mixture on the assembly with graphite or boron nitride.
In one embodiment of the present subject matter a novel device is provided comprising a quartz or glass body forming a chamber and having one or more pinch or shrink seals formed in the body, and a metallic foil positioned within the pinch or shrink seal, the metallic foil having a coating on at least a portion thereof comprising a refractory abhesive.
Another embodiment of the present subject matter provides a novel electrical lead assembly suitable for providing an electrical connection through a pinch seal in a quartz or glass body where the assembly includes a metallic foil having a coating on at least a portion thereof comprising a refractory abhesive. A further embodiment of the present subject matter provides a novel electrical lead assembly having a portion of metallic foil and an electrode or filament pin attached to said foil. An electrical lead may be attached to the foil, and a coating may cover at least a portion of the assembly, the coating having a refractory abhesive.
In a further embodiment of the present subject matter, a method is provided including the steps of providing an electrical lead assembly comprising a metallic foil and applying a protective layer comprising fusible glass precursors to at least a portion of the assembly. A layer of material may be applied over at least a portion of the protective layer, the material being suitable for preventing adhesion of the protective layer overlaid by the material and a glass body when the electrical lead assembly is sealed within a pinch or shrink seal in the body.
Another method of the present subject matter may include the steps of providing an electrical lead assembly comprising a metallic foil and applying a protective layer to at least a portion of the assembly, the protective layer comprising fusible glass precursors and a material which prevents mechanically strong bonding of the protective layer to a glass body when the electrical lead assembly is sealed within a pinch or shrink seal in the body.
One novel electrical lead assembly according to an embodiment of the present subject matter includes a metallic foil having one or more leads attached thereto, and a protective layer on at least a portion of the metallic foil, the protective layer comprising one or more fusible glass precursors. The assembly may also include a layer of material overlaying at least a portion of the protective layer, the material being suitable for preventing adhesion of the protective layer overlaid by the material and a glass body when the electrical lead assembly is sealed within a pinch or shrink seal in the body.
It will be noted that although the present invention is illustrated with these and other objectives, that the principles of the invention are not limited thereto and will include all applications of the principles set forth herein. These and other objects can be realized by simultaneous reference with the following non-exhaustive illustrative embodiments in which like segments are numbered similarly.